In a heat exchanger functioning as a condenser or an evaporator of a refrigeration cycle apparatus such as an air-conditioning apparatus or a refrigeration apparatus, when a refrigerant flow passage therein is divided into a plurality of paths, a refrigerant distributor that distributes refrigerant to each path is necessary at the entrance of the heat exchanger.
For example, in a multi-type air-conditioning apparatus in which a plurality of outdoor units or indoor units are connected in parallel, a refrigerant distributor is necessary to distribute refrigerant from a main refrigerant flow passage to each unit.
Such a refrigerant distributor is desired to perform distribution to a plurality of paths more equally and with less unevenness from the viewpoint of further performance improvement of an air-conditioning apparatus. In recent years, aluminum has been increasingly used in air-conditioning parts from the viewpoint of product weight reduction and improvement in cost-performance ratio based on material workability.
When heat transfer tubes of a heat exchanger are copper pipes, a distributing portion of a refrigerant distributor is formed of copper or brass by shaving processing, and outflow pipes and an inflow pipe are formed of copper. The outflow pipes are brazed to the distributing portion, the inflow pipe is brazed to the distributing portion, and the outflow pipes are brazed to heat transfer tubes of the heat exchanger.
In a conventional refrigerant distributor 1, the heat capacity of outflow pipes 2 is small, and the heat capacity of a distributing portion 3 is large as shown in FIG. 8. Therefore, the heat capacity difference is large, and, when joining both members by burner brazing, temperature control is difficult, and brazability is not stable. To solve this problem of burner brazing, from the viewpoint of improvement in reproducibility of heat input, a high-frequency induction heating coil is commonly used as a brazing heating unit in the production site of a refrigerant distributor (especially made of copper or brass).
When the heat transfer tubes are made of aluminum, the distributing portion 3 of the refrigerant distributor 1 is formed of aluminum by shaving processing, and the distributing portion 3, the outflow pipes 2, and the inflow pipe 4 are also made of aluminum. The outflow pipes 2 are brazed to the distributing portion 3, and the inflow pipe 4 is brazed to the distributing portion 3.
On this occasion, in aluminum brazing, the melting point of brazing filler metal is about 580 degrees C., whereas the melting point of base material is about 650 degrees C., and the difference between the melting point of brazing filler metal and the melting point of base material, that is, the allowable temperature range is as small as about 70 degrees C., a fraction of that in copper brazing. Therefore, when performing joining by burner brazing, the heat capacity of the distributing portion 3 having a solid cylinder structure is large, temperature unevenness is likely to occur between the radially inner and outer parts, the allowable temperature range is partially exceeded, the base material melts, on the other hand a region where brazing filler metal is unmelted is formed, temperature control is difficult, and brazability is worsened. When a high-frequency induction heating coil is used, the reproducibility of heat input improves. However, because high-frequency current flows mainly at the surface of the work due to skin effect, heating is local, and, in the case of aluminum, base material is likely to melt.
That is, when joining a distributing portion 3 of an aluminum refrigerant distributor and outflow pipes 2, there is a problem in that, because the number of the outflow pipes 2 is large, the difference in melting point between brazing filler metal and base metal is small, and the heat capacity difference between the outflow pipes 2 and the distributing portion 3 is large, it is difficult to secure highly reliable brazing.
So, hitherto, in particular, the joining of outflow pipes 2 and a distributing portion 3 different in heat capacity has been performed by furnace brazing to eliminate cumbersomeness of temperature control (see, for example, Patent Literature 1).
Because a distributing portion 3 of a refrigerant distributor 1 is formed by shaving processing, in the case of aluminum, there is also a problem in that, because machinability is poor and machining takes time compared to copper or brass, processing cost is high.